Field of the Invention
The present invention relates to an organic electroluminescent device (OELD). In particular, the present invention relates to an OELD that can improve a capacitance of a storage capacitor and an aperture ratio of a pixel region, and implement a high resolution.
Discussion of the Related Art
Recently, facing information society, display field of displaying electric information signals has been rapidly advanced, and accordingly, various flat display devices have been developed and used.
As flat display devices, a liquid crystal display device (LCD), a plasma display panel device (PDP), a field emission display device (FED), an electroluminescent display device (ELD) and the like are used. Because these flat display devices have excellent performances of thin profile, lightweight and low power consumption, and the display devices have been rapidly substituted for convention cathode ray tubes (CRTs).
Among the flat display devices, organic electroluminescent devices (OLEDs) have properties of high brightness and low driving voltage.
Further, the OELD can have high contrast ratio and very thin profile because of self-luminescent device, is easy to display moving images with response speed of about several microseconds, has no limit of viewing angles, is stable at low temperature, and has driving circuit easily manufactured and designed because of low driving voltage of DC 5V to 15V.
Accordingly, the OELD having the above advantages are used for various IT devices such as TV, monitor, mobile phone or the like.
The OELD is categorized into a passive matrix type and an active matrix type. A passive matrix type OELD has scan lines and signal lines crossing each other to form devices in a matrix form, and the scan lines are sequentially driven according to times to drive each pixel, and thus in order to realize an average brightness as required, an instant brightness as many as the average brightness multiplied by a number of lines is needed.
However, in an active matrix type OELD, a thin film transistor (TFT) as a switching element to turn on/off a pixel is formed in each pixel region, a first electrode connected to the TFT is turned on/off in each pixel region, and a second electrode facing the first electrode becomes a common electrode.
Further, in the active matrix type OELD, a voltage applied to the pixel region is charged in a storage capacitor and maintained until a signal of a next frame is applied, and thus each pixel continues to be driven for one frame irrespective of a number of scan lines.
Accordingly, even though a low current is applied, the same brightness is emitted, and thus there are advantages of low power consumption, high precision and large size. Thus, recently, the active matrix type OELD has been widely used.
FIG. 1 is a cross-sectional view illustrating an OELD according to the related art.
For the purpose of explanations, a region of a pixel region P where a switching TFT (not shown), a driving TFT DTr are formed is referred to as an element region DA, and a region of the pixel region P where a storage capacitor StgC is formed is referred to as a storage region StgA.
With reference to FIG. 1, a semiconductor layer 13 which includes a first region 13a of an intrinsic poly silicon and second regions 13b doped with impurities, a gate insulating layer 16, a gate electrode 21, an inter-layered insulating layer 23 having a semiconductor contact hole 25 exposing each second region 13b, and source and drain electrodes 33 and 36 are sequentially located on a first substrate 10 to form a driving TFT DTr, and the source and drain electrodes 33 and 36 are connected with a power line (not shown) and a organic light emitting diode E, respectively.
The organic light emitting diode E includes first and second electrodes 47 and 63 facing each other, and an organic light emission layer 60 therebetween. The first electrode 47 contacts an electrode of the driving TFT DTr in each pixel region, and the second electrode 63 is formed entirely on the organic light emission layer 60.
A storage capacitor StgC is formed in each pixel region to maintain a video signal until a next video signal is input.
Regarding a structure of the storage capacitor StgC, a first storage electrode 15 made of doped poly silicon is formed at the same layer as the semiconductor layer 13, the gate insulating layer 16 functioning as a dielectric layer is formed on the first storage electrode 15, and a second storage electrode 18 made of the same material as the gate electrode 21 is formed on the gate insulating layer 16, thereby forming a first storage capacitor StgC1.
Further, the inter-layered insulating layer 23 is formed on the second storage electrode 18, a power line (not shown) is formed on the inter-layered insulating layer 23, and a part of the power line on the inter-layered insulating layer 23 forms a third storage electrode 38. Accordingly, the second storage electrode 18, the inter-layered insulating layer 23 and the third storage electrode 38 forms a second storage capacitor StgC2.
Accordingly, the related art OELD 1 includes the first storage capacitor StgC1 and the second storage capacitor StgC2 connected in parallel with each other, and has a storage capacitance which is a sum of a storage capacitance of the first storage capacitor StgC1 and a storage capacitance of the second storage capacitor StgC2.
Recently, a high resolution of a display device has been rapidly required.
A resolution of a display device is defined as a PPI (pixel per inch), a high-resolution display device means a display device having 300 PPI or more. Further, recently, a display device having a super resolution of 500 PPI or more is required.
To realize a super resolution, a number of pixel regions per a unit area should increase, and this means that a size of each pixel region is reduced.
When a size of a pixel region is reduced, components constituting the pixel region are also reduced, and thus an area of a storage capacitor is reduced. This causes reduction of a storage capacitance.
Further, when a size of a pixel region is reduced, a size of an organic light emitting layer is reduced, and a storage capacitance is reduced accordingly but is not proportional.
In other words, because a reduction of a storage capacitance due to a size reduction of a storage capacitor is greater than a reduction of a pixel region, an area to form a storage capacitor in a pixel region is required to more increase.
Further, when an area of a storage capacitor increases in a pixel region, an area of a storage capacitor more increases relatively, and thus aperture ratio is reduced.
A flexible OELD has a flexibility using a plastic film instead of a glass substrate, thus the flexible OELD has light weight and high shock resistance, and can be manufactured in various forms because the OLED can be curved or bent. Accordingly, a flexible OELD has been researched recently.
However, a flexible substrate made of a plastic film is weak to contact with moisture and oxygen compared with a glass substrate. Accordingly, due to moisture and oxygen gradually permeating the flexible substrate, a flexibility of the flexible substrate is damaged or an internal circuit of the flexible substrate is damaged.
Further, the flexible substrate is prone to be charged due to static electricity, and due to an induced electric field, a TFT malfunctions and display quality is degraded thus.